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1991 300SL
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132 Posts
Discussion Starter #1
I noticed that my cabin air fan doesn't seem to be working on high speed. If I leave it in auto and set it on low or manually advance the speed I get a steady progression from 1 through 3 but going to 4 doesn't seem to increase it any further (1991 300SL)? Pretty sure the air flow I am getting is equivalent to setting 3 versus 4 as it is only "adequate" in hot wheather. If I missed this in the sitckys or my various attempts at searches please just tell me where to look for the answer?

Does the fan have a separate power supply / fuse for the highest speed? I would have thought the regulator would simply cut out any and all resistance and put full voltage to the motor on high?

BTW, I cannot recall if it worked on high before the recent AC evaporator / expansion valve / compressor / dryer replacement. Blows cold but not enough air flow to intially get the temp down on hot days. All the vent controls work as well as the center vent "extra cold" button and other than this the climate control unit seems to be fully functional.
 

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SL55, 300SE, GL450, Daytona 955i
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look up blower regulator.
 

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94 SL600, 89 Jag XJS conv, 85 Daimler Princess, 97 993 63 Avanti 16 Lincoln MKZ hybrid 16 Silverado
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720 Posts
the power supply comes in one set of wires. I suspect one fuse. The wire for the blower motor does come from a metal, cooler finned structure that sit upon the fan housing. when I was redoing the fan motor clamp, I also lubricated motors, placed CPU paste on the thermocouple surface and cleaned the fins of the fan. The blower related speed changes ,as before, but the temp. sensitivity seems improved. Could this be connected to your lack of 4th notch fan speed?
 

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1991 300SL
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132 Posts
Discussion Starter #4
look up blower regulator.
Bayhas,

Thanks for the search tip. It all comes down to terminology doesn't it! The related posts look like fertile ground (no pun intended):) Guess I'll be stripping away the wiper assembly again this weekend and trouble shooting starting with volt readings either side of the resister / regulator.
 

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94 SL600, 89 Jag XJS conv, 85 Daimler Princess, 97 993 63 Avanti 16 Lincoln MKZ hybrid 16 Silverado
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720 Posts
A second observation relates to the air flow. (aam's reply) If the filter is dirty or the "outside" baffel does not open then the airflow noise is less. and the 4th fan stage may not sound like the expected increase. Good luck with all the small parts and threaded plastic rivets! My old hands have let more that a few things slip away!
 

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1991 300SL
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132 Posts
Discussion Starter #7
Have you replaced the cabin filter????

Regards,
aam.
With all due respect, no I left the totally clogged black moldy one in the car when I did the AC evaporator to save $9 (not!!!):) Yes it has a nice new cabin air filter.
 

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1991 300SL
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132 Posts
Discussion Starter #8
Well the good news is that what I tried today failed!:) I have a friend with an identical car and I swapped out the climate control unit with no change in results!!:):) That is a good thing as they are hundreds of dollars and it looks like the resistors can be had (minus the heat sink) for about $32 bucks.

The down side is of course that the labor to get to and test the blower motor power supply is not minor. Nothing I haven't done recently (having done the AC evaporator recently) but a PITA.

So, the couple of hundred dollar question (assuming it proves to be the resistor unit) is has anyone used the $35 or so resistor unit only like they sell on eBay with their old heat sink successfully? It is a whole lot cheaper than the whole unit from anyone.
 

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Greek God of the R129
SL500-500SEL-190E
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With all due respect, no I left the totally clogged black moldy one in the car when I did the AC evaporator to save $9 (not!!!):) Yes it has a nice new cabin air filter.
My job was mostly trouble shooting... not on cars.
I would start with simple questions.
You will be amazed of the finds..... Lol.

Regards,
aam.
 

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1991 300SL
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132 Posts
Discussion Starter #10
All fixed!:) It was indeed the blower motor resistor. The $32 unit off eBay was a perfect fit and works like a champ (hopefully for a long time). Thanks to all who pointed me in the right direction. Cold air WITH lots of air movement!!!! Yipee!!!:)
 
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